1. Field of the Invention
The invention is directed to a method of treating the waste rinse acid from aluminum bright-dipping operations or possibly from wet phosphoric acid processes to produce monoammonium phosphate and other chemicals therefrom.
2. Related Art
Bright-dipping is a commonly used process in industry for chemical polishing of aluminum, and commercial bright-dip operations utilize a solution based mainly on phosphoric acid. In the bright-dip process, phosphoric acid with additives (diammonium phosphate as a fume suppressant and perhaps copper sulfate and/or nickel sulfate) are used by piece and continuous coil aluminum finishers to polish the surface of the aluminum prior to sulfuric acid anodizing. In the bright-dip process, cleaned aluminum parts are immersed in the bath containing phosphoric acid before being transferred to the rinse. With a bright-dip bath, the bath constituents adhere to the parts being polished and are constantly being transferred to the subsequent rinse tank. It is a wasteful process since only 10% to 15% of the purchased phosphoric acid added to the bright-dip bath is actually consumed in chemically polishing the surface of the aluminum. The remainder is lost to the subsequent rinses. In addition to being an expensive operation, commercial bright-dip processing also creates a serious problem in the handling and disposal of the waste liquor because phosphate is one of the more objectionable materials from the standpoint of stream pollution.
Chemical methods for the neutralization and treatment of the rinse acid have been developed, but none of these have achieved widespread acceptance because of the cumbersome equipment required and the high initial cost thereof and the large quantities of treating chemicals consumed. One initial method of recovery widely used employs countercurrent rinsing where sufficient rinse tanks are operated in such a way that water flow is countercurrent to the flow of the parts to control the first rinse acid concentration in the range of 30%-40% H.sub.3 PO.sub.4. Rinse acid over 30% phosphoric acid is sold directly to liquid fertilizer manufacturers as a raw material but the price is generally low and storage facilities are required due to the seasonal nature of the fertilizer business. A refinement of this recovery for the fertilizer industry is evaporative recovery where the rinse acid is concentrated by evaporation to high strength for resale, for example, 75%. Although the 75% acid requires less storage space and costs less to ship than 30% rinse acid, recovery of 75% acid requires additional capital and operating costs.
Various other techniques have been proposed to recover the rinse acid. For example, a process to recover the phosphoric acid for reuse is that of ion exchange where cation resins exchange hydrogen ions for aluminum and other metal cations in the phosphoric acid, which is then concentrated to the required strength for reuse in the bright-dip bath. The consumption of the regenerent for the ion exchange resin (e.g., sulfuric acid) is high, consequently this technique is not widely utilized.
Another proposed recovery is by adsorption and desorption technique. The principle of the separation is called acid retardation. When a mixture of strong acid and its salts is passed through a column of base and an ion exchange resin in a common ion form, the movement of the acid on the resin bed is retarded (i.e., slows down) relative to the movement of the salt. This technique could theoretically be used to separate aluminum phosphate from the phosphoric acid in the rinse acid and the recovered phosphoric acid could then be concentrated by evaporation for use in the bright-dip bath. Because of the insolubility of aluminum phosphate, AlPO.sub.4, in water, free phosphoric acid is required to hold the aluminum phosphate in solution. This not only reduces the efficiency of the recovery but also increases waste disposal problems. The principle advantage of the acid retardation technique compared to the cation exchange method is that the acid can be desorbed from the resin by washing it with plain water. However, this system also requires high capital investment, has high operating and maintenance costs and has not received commercial acceptance.
U.S. Pat. No. 4,044,106 discloses a process for reclaiming phosphate from bright dip rinse acid. The process comprises reacting the acid with soda alkalies to yield trisodium phosphate, sodium aluminate, sodium nitrate and insolubles. The solution is further treated to recover the trisodium phosphate.
U.S. Pat. No. 4,377,561 discloses a process for the clarification of "black" phosphoric acid produced via the wet process by mixing with bright dip rinse acid.
Monoammonium phosphate can be utilized in fire extinguishers and for food uses. Monoaluminum phosphate solutions are utilized as binders in refactories and diammonium phosphate is utilized as a flameproofing agent, in food applications, and in the production of halophosphate phosphors.